The ability to assemble an electronic circuit from various discrete components or integrated circuit chips requires that they be properly connected for electrical conductivity and fixed into position for long term mechanical integrity. Absent such good, long term mechanical stability and electrical conductivity the reliability of any circuit is seriously degraded.
Current fabrication techniques involve the use of electronic packaging substrates, such as printed circuit wiring boards that electrically connect and mechanically support various electrical elements, including discrete components (such as resistors, transistors, diodes, switching arrays, etc.) integrated circuit components (such as memory devices, microprocessors, multiplexers, etc.), and various other circuit elements (such as transformers, connectors, heat sinks, etc.). The contacts between the printed circuit board and the electrical device must have physical, chemical, electrical and mechanical integrity and stability.
Soldering is the primary process used for connecting the electrical elements to the printed circuit board and fixing them in position. Soldering involves the use of a low melting point metal alloy, usually of the lead-tin type, that will join metals together at temperatures around 450.degree. F. In one technique, metal pads are formed on the exposed surface of the printed circuit board and receive a coating of solder, or a small solder bump. The electrically conducting leads on the circuit component are brought into contact with the metal pads, heat is applied to raise the temperature of the pads and leads, and the solder is then heated to reflow and join the leads and pads. When the solder cools it resolidifies, thereby providing mechanical strength and a unified electrical connection. However, most metallic surfaces, including the solder, metal pads and leads, will oxidize in normal use, hindering the soldering process or even causing it to fail.
Most soldering processes include three basic steps: (1) pre-cleaning and deoxidation of the solder surface; (2) solder reflow and/or reflow joining; and (3) post-soldering cleaning. The solder reflow is critical. The reheated solder must flow freely between the circuit board pad and electrical lead to provide an air tight seal and a strong mechanical bond. However, proper reflow can occur only after any oxide coating is removed from the surface to be soldered because the high melting point oxides will interfere with or even prevent wetting of the two surfaces to be joined.
Pre-cleaning is conducted to remove impurities and oxide from the surface of the metal and solder that would create electrical resistance, weaken mechanical stability, or cause long term degradation of the union. Different flux materials are used in the pre-cleaning to prepare the surface and facilitate an unimpeded reflow of the liquid solder to perform an air tight and conductive bond between the pad and electrical lead. Activated fluxes, such as zinc, ammonium chloride, mineral acid-containing materials, and the like, are typically used in "coarse" soldering applications, i.e., repairing coarse wiring in motors or houses. Highly acidic fluxes are used for soldering aluminum layers. Aluminum has a tenacious oxide layer which is chemically very inert and difficult to remove. Fluxes used with aluminum can contain metal chlorides, fluorides, and ammonium compounds.
Unfortunately, the corrosive nature of the fluxes is becoming increasingly incompatible with the sensitive microelectronic assemblies. For most microelectronic devices, the standard practice is to reduce the acid activity of the flux to a mildly activated or non-activated grade in an attempt to minimize the adverse effects of the flux on the components. The problem is exacerbated by the shrinking size of electrical components and bonding pads, the growing use of surface mount technology, and the increasing demand for flip-chip device bonding.
The post-soldering cleaning step removes any flux residue remaining from the precleaning and deoxidation steps. However, as the size of electronic components has continued to decrease, the gaps between assembled parts and the risk of solidification cavities in solder joints has made it difficult to do post-soldering cleaning. Inefficient or ineffective cleaning can reduce the long term reliability of the entire assembly, causing higher defect levels and higher rework costs. In addition, the chemical activity and physical abuse brought about by the post-soldering cleaning can have its own negative impact. Although some cleaning equipment, newer materials, and refined processes solve some of the problems, there may still be undesirable effects and environmental concerns.
A fluxless soldering process can replace the precleaning step and virtually eliminate the post-cleaning step, however, it is still necessary to deoxidize the surfaces to insure complete solder reflow and bonding.
Various attempts at fluxless soldering have been made but high temperatures or pressures are required, or it is necessary to provide external energy or stimulation to enable or catalyze the deoxidation process.
For example, P. Moskowitz, et al., J. Vac. Sci. Tech. 4, (May/June, 1986) describes a dry soldering process for solder reflow and bonding of lead-tin solder using halogen containing gases for the reduction of the surface oxide to facilitate solder reflow. This process requires the use of a platinum catalyst mesh in a vacuum chamber with a temperature in excess of 300.degree. C., which can damage delicate electronic components.
IBM Technical Disclosure Bulletin 27 (April, 1985) describes the use of halogenated gases in an inert gas carrier at elevated temperatures to produce a reduction of solder oxides by the reactive gas and to allow solder reflow. This process requires high temperatures.
P. Moskowitz, et al., J. Vac. Sci. Tech. 3 (May/June, 1985) describes a laser-assisted fluxless soldering technique for solder reflow. However, laser radiation is required to excite an otherwise non-reactive gas in the presence of a preheated solder surface. This technique requires direct access of the laser beam to the solder surface, thus limiting the applications as well as resulting in a low throughput process.
U.S. Pat. No. 4,921,157 discloses a fluxless soldering process for semiconductor devices. Solder surface oxides are removed in a fluorine-containing plasma assisted process before it is reflowed.
German Patent No. 3,442,538 discloses a method of soldering semiconductor elements where a semiconductor element having an aluminum layer is subjected to a fluorine-containing plasma. The treated aluminum surface is then contacted with a soft solder. Process conditions include treating the aluminum layer with a fluorine-containing plasma for about one hour in a vacuum at a temperature of about 197.degree. C. to 397.degree. C. Thus, high temperatures are required, and throughput is limited.
U.S. Pat. No. 4,646,958 describes a fluxless soldering process using silane (SiH.sub.4) at elevated temperatures of about 350.degree. C. to about 375.degree. C.
U.S. Pat. No. 4,821,947 describes a wave soldering process which is performed in an inert atmosphere at temperatures below 300.degree. C., but a reflow process is not described.
U.S. Pat. No. 4,919,729 discloses a soldering paste which may be used in a reducing atmosphere to eliminate flux. However, a reducing atmosphere of hydrogen, heated to approximately 300.degree. C. to 500.degree. C., is required.
U.S. Pat. No. 4,937,006 describes the use of a gas, heated to a temperature sufficient to melt solder, which is directed at the molten solder at a momentum sufficient to disperse the oxide layer at the surface of the molten solder. This dispersion allows the oxide layer to wet the solder wettable surface.